A BS digital broadcasting receiver includes a synchronous detection circuit for detecting PSK modulated waves, and in order to create reproducing carrier signals synchronized with reception signal carriers to be used for synchronous detection, implements carrier reproduction with auto-correlation function. A prior art carrier reproducing circuit is configured as shown in FIG. 5.
A PSK modulated signal which was converted to comprise a medium frequency is supplied to two multipliers respectively which configure the synchronous detection circuit 1 and are multiplied by cosine wave data and sine wave data with a multiplier and undergoes synchronous detection. Multiplication output I data as well as Q data outputted from the synchronous detection circuit 1 are supplied to two digital low pass filters respectively which configure a digital low pass filter 3 and the high digit frequency components which are being respectively outputted from the synchronous detection circuit 1 are removed so that the I data as well as the Q data being baseband signals are sent out.
An output baseband signal from the digital low pass filter 3 includes as shown in FIG. 6 a first to a forty-eighth slots transmitting a TMCC section as well as information to the head of one frame. A TMCC is a Transmission and Multiplexing Configuration Control signal and transmits slot number information to designate transmission method (designate modulation method or error correction code substituting ratio) as well as to control time slots so as to correctly decode bit information at the phase point of demodulation with the TMCC information. A TMCC section is a period of time during when the TMCC signal is sent. The I data as well as the Q data are supplied to the Transmission and Multiplexing Configuration Control signal (TMCC) section detecting circuit 4 so that the TMCC section is detected in the TMCC section detecting circuit 4 and the signal showing the TMCC section width (192 symbols) is outputted.
On the other hand, the I data as well as the Q data being baseband signals outputted from the digital low pass filter 3 are supplied to a signal point arrangement converting circuit 5 and are converted into a signal point position signal based on the I data as well as the Q data being output base band signals from the digital low pass filter 3. The signal point position signal subject to conversion in the signal point arrangement converting circuit 5 is supplied to a phase detector 6 and undergoes phase detection.
The phase detection output from the phase detector 6 is supplied to the auto-correlation function determining circuit 7 together with the above described TMCC section width signal so that the auto-correlation function is obtained over the TMCC section width from the phase detection output and the delayed phase detection output subject to delay for time τ on the phase detection output. A signal based on the period of the obtained auto-correlation function wave-form represents a shift of the oscillation frequency of the NCO 2 from the carrier frequency, and this signal is supplied to the numerical control oscillator (NCO) 2 from the auto-correlation function determining circuit 7. In the NCO 2, the signal based on the period of the auto-correlation function wave-form outputs cosine wave data as well as sine wave data of the reproducing carrier signal having the frequency synchronized with the carrier from the NCO 2 which are supplied to the multiplier of the synchronous detection circuit 1 and are multiplied by the I data as well as the Q data so that carrier reproduction is implemented.
Here, as described above, involvement of method to detect the auto-correlation in the carrier reproducing circuit is known to be strong against noises.
The frame configuration of the BS digital broadcast has as shown in FIG. 6(a) at its head following a frame synchronization (not shown) the header information modulated with BPSK called TMCC and the TMCC section is formed with 192 symbols.
Here in the case where the oscillation frequency in the NCO is shifted from the carrier frequency, the phase detection output of the TMCC signal in the TMCC section will become a sawtooth wave a as shown with broken lines in FIG. 6(b). When the C/N is sufficiently high, a beautiful sawtooth wave as shown with broken lines a is reproduced. In addition, since the period of this sawtooth wave represents a shift frequency of the oscillation frequency of the NCO 2, the differential coefficient as well as the period of the sawtooth wave a can be measured directly. However, when C/N is low, signals based on noises are multiplexed onto the sawtooth wave a due to noises to give rise to the one shown in the solid line wave-form b in FIG. 6(b), and as for differential coefficient as well as period, it will become impossible to measure its period T directly from the wave-form b.
Especially, when the phase of the signal point position signal is near 90 degrees, for example, in the case of position designated with A in FIG. 6(b) and FIG. 5, the detection phase will exceed 90 degrees even with a tiny noise component, but since a signal exceeding +90 degrees is detected as −90 degrees, an enormously large detection error will be given rise to. FIG. 7 shows constellation of a signal point position signal and the oblique line portion shows an uneven range of the signal point position signal.
Therefore, a signal wave-form b including noises is not directly measured to measure oscillation frequency shift of the NCO 2, but with auto-correlation function, noises are reduced. In the case where the input signal is a period function, the auto-correlation function will become a period function with the same period. Since this auto-correlation function is a signal processing which is strong against noises, the period of the input signal can be correctly obtained from this auto-correlation function also in the case where noises exist. Accordingly, the phase detection output of the wave-form b in FIG. 6(b) will not be measured for its period directly, but the auto-correlation function is obtained and the period of its wave-form is measured.
FIG. 8 is an explanatory view of calculation of an auto-correlation function as well as of its wave-form. The phase detection output wave-form from the phase detector 6 is as shown in FIG. 8(a), where the wave-form b in FIG. 6(b) was redescribed to be represented by θ(t), and FIG. 8(b) shows wave-form θ(t+τ) subject to delay time τ from the wave-form in FIG. 8(a), where auto-correlation function Φ(τ) is calculated over the balance section by subtracting the delay time τ from the TMCC section detected by the TMCC section detecting circuit 4. In FIG. 8(b), it is described as an arithmetic operation section. The auto-correlation function Φ(τ) is expressed in an equation to become the one shown in the following equation (1):Φ(τ)=Σ{θ(t)−θave}{θ(t+τ)−θave}  (1)
In the equation (1), θ(t) denotes a phase detection output with an adding section being an arithmetic operation section from 0 to (M−1−τ). Here, reference character M denotes a symbol number of an observation section, that is, a symbol number of a TMCC section, and in the BS digital broadcast the symbol number of the TMCC section is 192. θave denotes an average value within the observation section of the phase detection output. The operated auto-correlation function is shown in FIG. 8(c). In a portion of a predetermined amplitude level of this auto-correlation function, zero cross of the auto-correlation function Φ(τ) wave-form is obtained to obtain an average period T.
An average period T is the average period T=π/ω, wherein ω is an angular velocity of alienation frequency, and here the alienation frequency ω denotes a shift between the oscillation frequency (reproducing carrier frequency) of NCO 2 and the carrier frequency. An alienation frequency is also described as a shift frequency. The angular velocity ω is obtained from the average period T and is supplied to the NCO 2 so as to give rise to a sine wave and a cosine wave of the angular velocity ω in the NCO 2 to be sent out to the synchronous detection circuit 1 so that carrier reproduction is implemented.
However, with the above described prior art carrier reproducing circuit, there is a problem that the direction of frequency shift, that is, polarity cannot be detected. That is, with this method of obtaining an auto-correlation function, for any of shift of oscillation frequency of the NCO 2 from the carrier which could be +Δω or −Δω, wave-forms of the auto-correlation function being an output of the auto-correlation circuit 7 is the same, and therefore alienation frequency is required to undergo polarity judgment, but polarity judgment cannot be implemented.
In order to avoid the issue of polarity judgment on the alienation frequency, there is a possibility that the oscillation frequency of the NCO 2 is shifted in advance to the initial state at the time of synchronous detection. With the frequency for shifting being made to be α, if this α is set at a value not less than the expected maximum alienation frequency of the NCO 2, then for the alienation frequency ω not more than that, the direction of polarity is determined to one. That is, as shown in FIG. 9(a), with the reproducing carrier frequency of the phase detection output being set at the center of the expected maximum alienation frequency range of the NCO 2, polarity judgment cannot be implemented.
Nevertheless, as shown in FIG. 9(b), with the reproducing carrier frequency being set at the minimum frequency of the expected maximum alienation frequency range of the NCO 2, polarity being negative will not take place (, that is, the oscillation frequency of the NCO 2 is always higher than the reproducing carrier frequency), the polarity is positive, and in this relation, it will not take place that polarity judgment cannot be implemented. However, the range in which the TMCC section is detected is the expected maximum alienation frequency range shown in FIG. 9(a) with the reproducing carrier frequency at the center, and therefore, direct application of this method to a carrier reproducing circuit in a BS digital broadcasting receiver will give rise to a problem that a portion where any TMCC section cannot be detected or in this example a crosshatched portion in the rightward half from the dotted line in FIG. 9(b) will be generated.
Accordingly, since the auto-correlation function at the time of carrier reproducing in a BS digital broadcasting receiver is operated based on the TMCC section, an indispensable condition is that the TMCC section can be detected, and when no TMCC section will become detectable, no auto-correlation function will be given by operation.
As another problem different from the polarity judgment of alienation frequency, when the alienation frequency Δω becomes too much small, the period of auto-correlation function T=π/ω will increase, giving rise to a problem that one period of auto-correlation function will not be contained within a TMCC section at a constant period and the period T cannot be obtained so that carrier reproduction cannot be implemented.
Objects of the present invention are to provide a carrier reproducing circuit capable of judging the polarity of alienation frequency in a carrier reproducing circuit using auto-correlation function to implement carrier reproduction, and to solve the problem that carrier reproduction cannot be implemented when alienation frequency is small.